Add missing dependency

In 66d96c3f, we added a dependency on httplib2 but this patch assumes
cinder is using vars/repo_packages, which it isn't on 9.0.x.  This
commit simply adds httplib2 to the correct service_pip_dependencies in
inventory/group_vars/cinder_all.yml.
Merge pull request #297 from mattt416/cinder_icehouse_fix

Add missing dependency